emit informative message if libz can't be found

In case libz can't be loaded we emit a message which will help
the user find the library.

Are there user visible changes in this commit?  no

Is bootstrapping needed?   no

Are tests included for new features?  no new features

Tests run:  none

Change-Id: I348241ee5a14b0425fbab91bcf9f6bcf2a2facfe

deflate.cl

@@ -69,8 +69,16 @@
 
 (defvar *libz-dll-loaded* nil)
 
+
 (if* (not *libz-dll-loaded*)
-   then (load (util.string:string+ "libz." sys::*dll-type*) :foreign t)
+   then (handler-case
+	 (load (util.string:string+ "libz." sys::*dll-type*) :foreign t)
+	 (error (c)
+		(error "~
+This module require the compression library named libz be present ~
+on the machine for the deflate module to load. ~
+See http://zlib.net/ for versions for various platforms.~% failure ~
+condition: ~a~%" c)))
 	(setq *libz-dll-loaded* t))